Is it possible to use just a catalyst 5k or 6k for an ATM backbone without having a LS 1010 in place?
Yes, for the 5k, it is basically a LS1010 on a module. This module can only go in slot 13. This runs the standard LS 1010 code, and can be a LEC, LECS, LES/BUS.
For the 6k, I "believe" you can only run a LANE module and be a LEC. It doesn't have an ATM backlane like the 5k.
The next step above the 5k for an ATM backplane would be an 85xx series.
Hope this helps,
CCNP, CCDA, CNE